FIG. 1 shows a run-out portion of a panel 1 which may form, for example, the skin of an aircraft wing or fuselage. The panel is reinforced by a series of elongate stringers which run along the length of the panel, one of which is shown in cross-section in FIG. 1. The stringer 2 comprises a web 3 extending at right angles to the panel, and a flange 4 engaging the panel 1.
The stringer 2 is terminated as shown. The web 3 is tapered near the termination in order to facilitate load transfer from the skin to the stringer, by providing a gradual increase in transverse bending and axial stiffness and relieving local stress concentrations. Furthermore, the flange 4 extends beyond the end of the web in order to render the stiffener tip more compliant and capable of following the panel when it is bent.
Pre-tensioned bolts 5 are used in order to prevent delamination or disbonding. As shown in FIG. 2, the pre-tensioning load 6 causes a portion 7 of the panel 1 and flange 4 to be in compression. However there is an end region 8 between the compressed region 7 and the tip 9 of the flange 4 which is not in compression. Therefore the bolts 5 cannot delay failure initiating at the interface 10 where the tip 9 of the flange 4 meets the panel 1.
An improved design is shown in FIG. 3. In this case a washer 11 is used to transfer the through-thickness compression region up to the tip of the flange, thus suppressing the peeling failure mode at the tip. However, the solution of FIG. 3 does not suppress sliding failure modes at the tip.